1. Field of the Invention
The present invention relates to a high-speed modulator and oscillator for microwave and milliwave use, and more particularly to a modulator which is superior in terms of high-frequency stability and which has a high degree of modulation.
2. Description of Related Art
Conventional modulators for microwave and milliwave use have been described in the literature (H. Mitsumoto, et al. xe2x80x9cGaAs FET Direct Frequency-Modulators for 42-GHz-Band HDTV Radio Cameras and 7-GHz-Band Field Pick-up Transmittersxe2x80x9d, IEEE MALE TYPE TERMINAL-S Digest, pp. 663-666, 1993).
The structure of the modulators disclosed in the above literature is shown in FIG. 5. The modulator 100 comprises an oscillating circuit 102 and a resonator portion 104. The resonator portion 104 comprises a reflective circuit board 106, a coupling line 108 which is formed on the surface of this board 106, a coupled load 110 which is coupled to one end 108a of the coupling line 108, a dielectric resonator 112 which is disposed on the surface of the board 106 so that said resonator is magnetically coupled with the coupling line 108, a xcex/2 line (xcex is the wavelength) 114 which is magnetically coupled with the dielectric resonator 112, and two varactor diodes 116 and 118 which are respectively connected to both ends 114a and 114b of the xcex/2 line 114. The xcex/2 line 114 is disposed in a position which is located on the surface of the board 106, and which is located on the opposite side of the dielectric resonator 112 from the abovementioned coupling line 108.
The oscillating circuit 102 supplies a negative resistance to the output port of the resonator portion 104 constituting the other end 108b of the coupling line 108 in the desired oscillation frequency band. As a result, the dielectric resonator 112 resonates, so that a high-reflection signal wave appears at the output port 108b from the coupling line 108. On the other hand, in frequency bands other than the desired oscillation frequency band, the dielectric resonator 112 does not resonate. Accordingly, a low-reflection signal wave appears at the output port 108b as a result of the action of the coupled load 110.
Furthermore, in this modulator 100, the dielectric resonator 112 is disposed on the upper surface of the board 106 between the coupling line 108 and the xcex/2 line 114. This dielectric resonator 112 is magnetic coupled with both the coupling line 108 and the xcex/2 line 114. Accordingly, the resonance frequency of the dielectric resonator 112 is affected by the xcex/2 line 114. Furthermore, varactor diodes 116 and 118 are coupled to both ends 114a and 114b of the xcex/2 line 114. Accordingly, the capacitances of the varactor diodes 116 and 118 vary according to the magnitude of the modulating wave that is input into the xcex/2 line 114, and as a result, the resonance frequency of the dielectric resonator 112 varies. Accordingly, the frequency and phase of the peak of the reflected wave that appears at the output port 108b vary according to the modulating signal. The oscillation frequency that is output from the oscillating circuit 102 can be modulated by this variation.
In this modulator 100 described in the literature, the frequency width of the oscillation frequency that is modulated is broadened as a result of the provision of two varactor diodes 116 and 118. Specifically, this modulator is advantageous in that a high degree of modulation can be obtained.
However, the following problems occur in the abovementioned modulator 100:
The dielectric resonator 112 is magnetically coupled with the coupling line 108 and xcex/2 line 114. Since the xcex/2 line 114 is formed by a strip line with a low resonance Q value, the Q value of the dielectric resonator 112 is affected by the Q value of the xcex/2 line 114 so that the Q value of the dielectric resonator 112 is lowered. As a result, the stability of the peak frequency of the reflected wave that appears at the output port 108b of the resonator portion 104 is lost. Accordingly, the stability of the oscillation frequency is also lost. This means that the modulation sensitivity drops.
Accordingly, an object of the present invention is to provide a modulator which has a high degree of modulation and a good modulation sensitivity.
Furthermore, a further object of the present invention is to provide a modulator which is capable of modulation at a stable oscillation frequency.
In order to achieve these objects, the modulator of the present invention comprises a resonator portion and an oscillating circuit which have the constructions described below. Furthermore, the resonator portion comprises a reflective circuit board, a coupling line which is disposed on a reflective circuit board, a coupled load which is coupled to one end of the coupling line, a dielectric resonator which is disposed on the reflective circuit board and which is magnetically coupled with the coupling line, a window portion which is formed in the undersurface of the reflective circuit board directly beneath the coupling line, a waveguide resonator which is disposed on the undersurface of the reflective circuit board in the area that includes the window portion, and which is magnetically coupled with the coupling line, and a varactor diode which is inserted between the opposite signal conductor surfaces of the waveguide resonator, and to which the input signal terminal is connected. Furthermore, the other end of the coupling line constitutes the output port of the resonator portion, and this output port is connected to the input port of the oscillating circuit. Moreover, the side of the coupled load that is not coupled to the coupling line is grounded.
The operating principle of the modulator constructed as described above is similar to that of a conventional modulator. In the desired oscillation frequency band, a negative resistance is supplied to the output port of the reflective circuit board from the oscillating circuit. As a result, so that a high-reflection wave appears at the output port from the coupling line. This magnitude and phase of this reflected wave are determined by the dielectric resonator, which has a high Q value. Furthermore, this reflected wave is also affected by the waveguide resonator that is magnetically coupled with the coupling line.
Specifically, both the dielectric resonator and the waveguide resonator are magnetically coupled with the coupling line. As a result, the resonance frequency of the dielectric resonator and the resonance frequency of the waveguide resonator coincide with the desired frequency. Accordingly, the impedance of the resonator portion with respect to the desired frequency (nearby frequencies) is increased. Consequently, the reflection to the coupling line is increased.
On the other hand, the magnetic coupling of the waveguide resonator and dielectric resonator to the coupling line is decreased with respect to frequencies that are not the desired frequency. As a result, the impedance of the resonator portion is also decreased, so that the coupling line shows impedance matching with the coupled load. Consequently, the reflection at the output port of the reflective circuit board is reduced, so that the oscillating circuit does not oscillate.
Thus, by making the resonator that causes coupled modulation with the dielectric resonator a waveguide resonator which has a Q value that is higher than that of the xcex/2 line, it is possible to maintain the Q value of the resonator portion in a high state. Accordingly, the oscillation frequency can be stabilized. Furthermore, the capacitance of the varactor diode of the waveguide resonator varies according to the modulating signal. Modulation is accomplished by means of this variation in capacitance. Accordingly, modulation at a stable oscillation frequency can be realized.